The present disclosure generally relates to apparatus and methods for treatment of spinal disorders using an intervertebral prosthesis which is disposed in an intervertebral disc space following removal of a damaged or diseased intervertebral disc.
The bones and connective tissue of an adult human spinal column consists of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex. Each tri-joint complex includes an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine includes the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis.
The spinal column is highly complex in that it includes these more than twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
Conventional intervertebral body cages generally comprise a tubular metal body having an external surface threading. They are inserted transverse to the axis of the spine, into preformed cylindrical holes at the junction of adjacent vertebral bodies. Two cages are generally inserted side by side with the external threading tapping into the lower surface of the vertebral bone above, and the upper surface of the vertebral bone below. The cages include holes through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior of the cage to incite or accelerate the growth of the bone into the cage. End caps are often utilized to hold the bone graft material within the cage.
The cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient's spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.
An objective in intervertebral disc replacement or intervertebral stabilization is to provide a prosthetic disc that combines both stability to support the high loads of the patient's vertebrae and flexibility to provide the patient with sufficient mobility and proper spinal column load distribution.
Numerous artificial intervertebral discs for replacing a part or all of a removed disc have been developed, namely, elastomer discs, ball and socket discs, mechanical spring discs and hybrid discs. Elastomer discs typically include an elastomer cushion which is sandwiched between lower and upper rigid endplates. The elastomer discs are advantageous in that the elastomer cushion functions similar in mechanical behavior to the removed intervertebral disc tissue. However, a disadvantage of this disc type is that the elastomer cushion experiences long term in-vivo problems stemming from microcracking, which detracts from its usefulness as a replacement option. Furthermore, attachment of the flexible elastomer cushion to rigid endplates presents additional difficulties. Examples of elastomer discs are disclosed in U.S. Pat. Nos. 5,702,450; 5,035,716; 4,874,389; and 4,863,477.
Mechanical spring discs usually incorporate one or more coiled springs disposed between metal endplates. The coiled springs may define a cumulative spring constant sufficient to maintain the spaced arrangement of the adjacent vertebrae and to allow normal movement of the vertebrae during flexion and extension of the spring in any direction. Examples of mechanical spring discs are disclosed in U.S. Pat. Nos. 5,458,642; and 4,309,777.
Ball and socket discs typically incorporate two plate members having cooperating inner ball and socket portions which permit articulating motion of the members during movement of the spine. The ball and socket arrangement is adept in restoring motion of the spine. Examples of ball and socket discs are disclosed in U.S. Pat. Nos. 5,507,816; and 5,258,031. These conventional ball and socket discs include some disadvantageous properties, such as the plate members not remaining interconnected (via the ball and socket) when tensile forces are applied. In other words, the ball is not captured within the socket. While there are existing captured ball and socket discs in the art, they are very complex in the manner in which the ball and socket are interconnected. An example of such a ball and socket configuration is disclosed in U.S. Patent Publication No.: 2003/0069643, the entire disclosure of which is hereby incorporated by reference.
Hybrid artificial intervertebral discs usually incorporate two or more disc types. For example, one known hybrid disc arrangement includes a ball and socket set surrounded by an elastomer ring. This hybrid disc provides several advantages with respect to load carrying ability, but, long term in vivo difficulties with the elastomer cushion remain a concern.
All of the above intervertebral devices suffer from one or more problems, such as being overly complex, difficult to assemble and implant, excessively limit the range of motion of the spine, and/or do not remain assembled in the presence of tensile forces. Thus, there are needs for a new intervertebral stabilizer.